The invention relates to an improved development process for forming relief structures based on radiation-crosslinked polymeric precursors of polymers which are resistant to high temperature and which are based on radiation-sensitive photoresists.
Photoresists for forming relief structures from polymers which are resistant to high temperature are used widely in the production of electrical and electronic components of etch resists and plating resists or of printing molds. One of the most accurate processes for structuring insulating materials and semiconducting and conducting materials in electrotechnology and electronics is the photo technique. This entails resist relief structures produced by the photo technique being copied on substrates by suitable processes, such as etching, vapour deposition and metallising non-electrically or electrically.
Moreover, resist relief structures can assume a permanent protective function, for example as an insulating material.
Processes for the production of relief structures of these types from polymers which are resistant to high temperature, and photoresists suitable for these purposes, and the photosensitive soluble polymeric precursors used for this are known, for example, from German Pat. No. 2,308,830, German Pat. No. 2,437,348, German Pat. No. 2,437,368 and German Pat. No. 2,437,422, and German Patent Application 3,227,584 and German Patent Application 3,233,912, corresponding to U.S. Pat. Nos. 3,957,512, Re. 30,186, 4,045,223, 4,008,489, and U.S. Patent Applications Ser. Nos. 516,399 and 531,781, respectively. The entire disclosures of these references are incorporated herein by reference.
As a rule, photoresists of the type described contain radiation-sensitive soluble polymeric precursors. They are applied to a substrate in the form of a layer or film, the layer or film is dried and then irradiated through a negative mask. This leads to crosslinking taking place on the irradiated areas and this drastically reduces the solubility of the material applied there. The nonirradiated parts of the layer or film are then removed by dissolving or detaching using a developer; the remaining relief structures can then be converted by heat treatment into polymers which are resistant to high temperature and which survive temperatures of 250.degree.-400.degree. C. without adverse effects on the edge sharpness and resolution.
The soluble polymeric precursors used are polyaddition or polycondensation products of polyfunctional carbocyclic or heterocyclic compounds having radiation-sensitive radicals with diamines, diisocyanates, bis(acid chlorides) or dicarboxylic acids as are described in the patents referred to above.
Particularly preferred soluble polymeric precursors are polycondensates of pyromellitic acid, which has two radicals which react to radiation and are bonded in the manner of esters to carboxyl groups, and a diamine which contains at least one cyclic structural element, such as, for example, 4,4'-diaminodiphenyl ether, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl sulfone or 2,4-diaminopyridine.
As a rule, soluble polymeric precursors of this type which are used in photoresists have molecular weights between 2,000 and 100,000, preferably between 4,000 and 60,000.
These soluble polymeric precursors are processed in a manner known per se to give the corresponding photoresists which, apart from a suitable solvent or mixture of solvents, can, where appropriate, contain additives which are known and customary in this technology, such as photosensitisers, photoinitiators, copolymerisable monomers or resins which react to radiation, adhesion promoters, plasticisers and pigments, dyestuffs and fillers.
The photoresists can be applied to the clean surface of the substrates at the layer thickness advantageous in the individual case by spraying, flow coating, rolling, spin coating and dip coating, the solvent then being removed by evaporation so that a radiation-sensitive layer remains on the surface of the substrate. The removal of the solvent can, where appropriate, be accelerated by heating the layer to temperatures up to 100.degree. C. Subsequently, the layer of photoresist is exposed to radiation which causes the groups which react to radiation to react so as to crosslink in the layer. The irradiation or illumination can be carried out through a mask, but it is also possible to guide a collimated beam over the surface of the radiation-sensitive layer. UV lamps are normally used for the irradiation, these emitting radiation of a wavelength of 200 to 500 nm and an intensity of 0.5 to 60 mw/cm.sup.2. An image pattern is now developed in the layer with exposure of parts of the substrate by treating the layer with a developer solution which removes the nonirradiated areas of the photoresist material. The resist images are obtained after customary washing and drying.
Now, the process step of development has particular importance in the production of radiatin-crosslinked photoresist structures since it exerts a considerable influence on the resolution and edge sharpness of the remaining resist structures as well as on their adhesion to the substrate and their material properties.
A good developer should remove the non-irradiated, soluble polymeric layers from the substrate in a short time which is suitable for practice. At the same time, the partial dissolution and erosion of the crosslinked resist layers, and the loss of thickness of the layer caused by this, should be kept to the minimum possible. In addition, constituents of the developer should penetrate into the crosslinked polymeric layers as little as possible since this is associated with swelling of the crosslinked resist layer and a decrease in its adhesion to the substrate. Furthermore, there is a requirement for a development process which is suitable for practice, that is to say the safety margin in the development time should be as wide as possible in that, while the erosion of crosslinked polymeric material is still within tolerable limits, the complete removal of non-crosslinked resist is ensured.
According to the state of the art, the developer solutions used are mixtures which consist of one or more solvents customary for the production of the photoresist and of a precipitant normally used in the production of the polymeric precursor of the photoresist. Examples of known and typical developer solutions are 4-butyrolactone/toluene, dimethylformamide/ethanol, dimethylformamide/methanol, methyl ethyl ketone/ethanol and methyl i-butyl ketone/i-propanol, each in the ratio 2:1 to 4:1, each of the substances mentioned first being solvents and each of those mentioned second being precipitants. After development it is customary to rinse with the precipitant, and this merely serves to remove the developer completely.
The disadvantages of developer solutions of these types according to the state of the art are that they always attack, to a greater or lesser extent, the crosslinked polymeric layers, and this is manifested by erosion of the layer, swelling of the layer and a diminution in its adhesion to the substrate, and by the safety margin in the development time being too narrow. Hitherto, in order to decrease the extent of attack, a precipitant has always been added to the developer fluid in addition to the solvent.